Oxidative degradation of monobasic carboxylic acids



United States Patent 3,251 878 OXIDATIVE DEGRADATION 0F MONOBASIC CARBOXYLIC ACIDS Joseph Z. Pasky, Oakland, Calif., assignor to Chevron Research Company, a corporation of Delaware No Drawing. Filed Jan. 2, 1963, Ser. No. 248,906

7 Claims. (Cl. 260--537) This is a continuation-in-part of co-pending application Serial No. 59,516, now abandoned, entitled, Preparation of Dibasic Acids, filed September 30, 1960.

This invention relates to. a novel method of oxidative decarboxylation. More particularly, this invention relates to a method of oxidative decarboxylation using oxygen and a polyvalent metal oxidation catalyst.

The oxidation of carboxylic acids to oxygenated products having fewer carbons has a wide Variety of applications. The possibility of degrading a carboxylic acid stepwise can be particularly useful in structure .determination of natural products and other compounds of unknown structure. By determination of the products obtained, one can usually reconstruct the carbon skeleton of the original carboxylic acid.

Oxidative decarboxylation can also be used in the preparation of acids or other oxygenated materials of lower carbon content than the original carboxylic acid. Common acids which are available naturally, or which are by-products and do not find useful application, may be degraded to more useful acids of lower chain length. When the carboxylic acid hasa carbon ring in it skeleton, the carboxylic acid may be degraded to dior polybasic acids, which find a variety of uses as monomers in condensation polymerization, crosslinking agents, etc.

In accordance with this invention it has now been found that at elevated temperatures in excess of about 50 C., monocarboxylic acids may be degraded to oxygenated products having fewer carbon atoms than the original material. The oxidation is carried out in the presence of oxygen and a polyvalent metal catalyst, e.g., cobalt. The reaction can be carried out with or without an inert solvent.

The carboxylic acids used in this invention have an alpha carbon which is not benzenoic. By this, it is intended that the alpha carbon is not an annular member of a; benzene ring or the alpha carbon maybe regarded as free of benzenoid unsaturation. Conversely, the alpha carbon is an aliphatic carbon, which may be saturated or unsaturated, i.e., ethylenic or acetylenic. The carboxylic acid should be monobasic and should have at least 4 carbons and preferably at least 6 carbons. The monocarboxylic acid may have bond-ed to the carboxyl group, acyclic, alicyclic, aralkyl, heterocyclic, etc., groups. It is preferred that the group be hydrocarbon. While there need be no limit to the maximum number of carbons of the monocarboxylic acids used in this invention, generallythe acids willnot exceed 24 carbons. It is preferred that they not exceed 16 carbons, and still more preferred they they not exceed 10 carbons.

Illustrative of the acyclic aliphatic acids which may be used in this invention are the following: n-butyric, isobutyric, valeric, caproic, heptylic, caprylic, pelargonic,

.,omega-chlorovale ric, omega-bromobutyric, lactic, capric,

3-decanoic lauric, myristic, palmitic, margaric, stearic, eicosanoic, tetraeicosanoic, etc. Among alicyclic acids, the following examples are illustrative: cyclopropane carboxylic acid, cyclobutane car- 3,251,878 Patented May 17, 1966 cyclooctanel carboxylic acid, etc. Also included are such terpenoid acids as abijetic acid, levopimaric acid, eburocoic acid, and such steroidacids as chloic acid, cholanic acid, 3-fl-hydroxy-eti-5-enic acid, etc.

Among aralkyl acids,'the following examples are illustrative: cinnamic acid, mandelic acid, 2-phenylcyclo-hexene carboxylic acid, fl-napht-hylpropionic acid, etc.

While hydrocarbyl carboxylic acids are preferred, such substituents as halogen, i.e., fluorine, chlorine, bromine and iodine, alkoxy, hydroxy, nitro, keto, cyano, etc., do not interfere with the reaction and may be present.

For the polyvalent heavy metal catalyst, various metals of atomic numberbetween 25 and 29 may be used. While cobalt is particularly preferred, manganese, iron, nickel and copper are operative. The catalyst may be introduced into the reaction medium in a variety of forms. For example, cobalt may be employed in the metallic form in a finely divided state as a powder, or in the form of a 'cobaltsalt. The salts of carboxylic acids, such as cobaltacetate, adipate, hexahydrobenzoate, propionate, stearate, and acetonate may be used with advantage. It is efiicient to use the salt of the acid to be oxidized. The inorganic cobalt compounds, 'such as cobalt carbonate,-cobalt oxide, cobalt hydroxide, etc., may also be used. It is preferred not tonse the cobalt halides. As in the case of cobalt, the salts ofthe various other metals, both organic' and inorgariic, are'also operable. In order to have the metal catalyst relativelyevenlydistributed throughout the medium, it is preferred that it be added inafinelydivided state. i

The oxygen may be introduced into the reaction medium by itself or in combination with other inert gases as, for example, air; Oxygen may be introduced into the reaction in a variety of ways. It may be bubbled through the medium, the medium may be rapidly agitated to maintain an equilibrium between the oxygen atmosphere and the medium or the oxygen-containing gas may be introduced as co-current or countercurrent flow in a. continuous method. Various' means knowniin' thezart may be used to increase the efficient medium. 'Whilethe reaction does not require a solvent, various solvents inert to the reaction may be used with advantage.

The solvent should permit solution of both the carboxylic I substrate," as 'well as the polyvalent heavy metal catalyst. A variety of solvents are operable. Illustrative of solv'en'ts which may be used arecarboxylic acids which are relativelyi'nert' under the reaction conditions, such as acetic, benzoic, toluic,'and their esters, such as phenylacetate, methylbenzoate, ethylbenzoate, "phenyltoluate, etc'. T

Irrespective of whether a solvent is used or not, the concentration of the catalyst should be at least 0.1% by 'weight based on carboxylic substrate. Customarily, the

amount of catalyst will be from about 0.1% to 1% by weight based on starting carboxylic acid but may "be as high as 10% or more; The upper limit is not critical, but is subject to handling considerations and cost. When a solvent is used, the starting material should be at least 5% by weight and obviously can be as high as Suband superatmospheric pressures may be used, but

solution ofthe oxygen into the From 3-t-butylcyclopentane carboxylic acid, S-t-butylglutaric acid is obtained. From l-perhydroindene carboxylic acid, 2-(2-carboxycyclohexyl)acetic acid is obtained, as well as hexahydrophthalic acid.

The time for the reaction can be varied widely depending on the monocarboxylic acid used as starting material and the product, analyzed by gas chromatographic 'methods. A yield of 58 mole percent of adipic acid based on.

unrecovered hexahydrobenzoic acid was obtained.

The following table demonstrates a number of examples carried out in accordance with the method described in Example I.

TABLE I Mole Percent HHBA, Solvent Catalyst, Temp, Time, Adipie Glutaric HHBA 2 Adi Ex. gins. Acetic CoAczAHzO, 0. hrs. Acid, Acid, Recovered, Aci in Acid, mls. gms. gms. gins. gIIlS. Unrecovered I HHBA 2 1 Rate of oxygen flow was 0.65 mole per mole of hexahyrlrohenzoic acid (HHBA) 1 per hour.

and the products desired. With increasing degradation, longer periods of time are necessary. Generally the reaction will require about 30 minutes, and can be carried out as long as hours, but will typically not exceed that amount of time. Of course, as the time is increased, further degradation of the products to acids having fewer carbons will result.

If it is desired to minimize the amount of degradation, as in the case for example of preparing adipic acid from hexahydrobenzoic acid, it is advantageous to add to the reaction medium a hydrocarbon hydrogen atom donor which is at least as active as cyclohexane in being able to give up a hydrogen atom to a free radical. It is well known that the hydrogen atoms of cyclohexane are much more active than the methylene groups of an n-paraflin, and almost as active as a tertiary hydrogen. The compounds that may be used as a hydrogen source, therefore, are such compounds as toluene, cumene, cyclohexane and alkyl-substituted cyclohexanes, diisobutylene, etc. By using from about 5% to.95% by weight based on monocarboxylic acidof the hydrogen vatom donor, the yield of product having only 1 carbon less than the monocarboxylic acid starting material is greatly enhanced. Such enhancement is particularly significant when the 'monocarboxylic acid, starting material is a cyclic material having the carboxy group bonded to an annular member.

Promoters may also be added to the medium to enhance the formation of the degradation products. Various materials may be usedas promoters, but particularly etfec' tive are the aldehydes, such as acetaldehyde, benzaldehyde, etc., and ketones, such as cyclohexanone. These may be added in relatively low concentration, customarily in the range of 0.05 to 0.5 mol per mol of monocarboxylic acid starting material.

The following examples illustrate the practice of the invention.

Example I 7 duration of the reaction. After a short-induction period,

carbon dioxide began to evolve and soon reached a value of 9% to 10% per hour based on total exit gas. The

reaction was continued for 5 hours. At the endof this time, the reaction product was fractionally distilled, removing starting material and other low boiling material and leaving a dibasic acid fraction.

The dibasic acid fraction was esterified with methanol In general, the yields of adipic acid are better at low conversions of hexahydrobenzoic acid. The extent of conversion was determined by measuring'the amount of CO evolution. The reactions herein described were usually carried to a conversion of 20% to 60% Example VII Example I was repeated except that 9.8 g. of manganese acetate tetrahydrate was used in place of cobalt as the catalyst for the reaction. The temperature was main:

tained at C. The reaction proceeded substantially as p in Example I.

Example VIII Substantially the same procedure as Example was Example IX.

Following the same procedure as Example I, except that 7.2g. of copper acetate was used as the catalyst, there was no CO evolution for the first 11 hours; but by 15 hours, therate of CO evolution was about 0.7% per hour. The rate held for the next 5 hours.

Example X Substantially the same procedure as Example I was followed, except that 142 g. of paramethylhexahydrobenzoic acid was oxidized to 3-methyl adipic acid in about 55 mole percent yield.

Example XI As previously indicated, the reaction can also be carried out by a continuous process.

To illustrate the continuous preparation of adipic acid,

a reactor was used comprised of a glass tube with a frittcd glass plug in the bottom to disperse the oxidizing gas; the OXldlZlng gas being introduced thereto below the fritted glass plug. The glass tube was further provided with an inlet tube at the top and an outlet tube positioned just above the sintered glass plug near the bottom of the reactor. Pumps were attached to each of these tubes for the introduction and removal of reactants and products.

A thermowell provided With a thermocouple extended well down into the reactor. The thermocouple was attached to a temperature controller which regulated the amount of heat input provided by a wire heater wound around the reactor. A gas outlet was provided for the exit gases, From the exit gases, the entrained liquids, e.g., water and acetic acid, were condensed and the gases were then passed through a flowmeter.

To the reactor was charged, in parts by weight, 420 parts of glacial acetic acid containing parts of cobalt acetate tetrahydrate and 128 parts of hexahydrobenzoic acid. The temperature was maintained at about 96 C. for about 5 hours after which the temperature rose to 97 to 99 C. for the remainder of the run. Oxygen was passed in at the rate of 1 mole per mole of hexahydrobenzoic acid per hour. The reaction was allowed to continue batchwise for about 5 hours at the end of which time the rate of CO evolution Was 6% per hour. At this point, a solution of the same ingredients at the same concentration as the original charge was fed in at a rate of about 140 parts per hour, while-at the same time the reaction product mixture was pumped out from the reactor at such a rate as to maintain a constant level of reactants in the reactor. Under these conditions, the r e of CO evolution leveled off at 7 to 8% per hour.

Samples were taken throughout the run, the composition of the sample remaining essentially the same throughout the run. After 32 hours, the reaction was terminated. The products were determined by vapor phase chromatography analysis, and yields based on unconverted hexahydrobenzoic acid were adipic acid, 48%; glutaric acid, 16%; and 30% of a mixture containing cyclohexanol, cyclohexanone and cyclohexylacetate.

Example XII The following is a list of the fatty acid products ob-.-

tained based on converted palmitic acid:

C 1.8, C 6.3, C 2.1, C 3.0, C 2.5, C 3.6,

Substantially the same procedure as Example I was followed, except that the feed to the reactor consisted of 102.4 g. of hexahydrobenzoic acid and 19.6 g. of cyclohexanone. at which. point a vigorous reaction began. It was necessary to cool the reactor in order to maintain a temperature of 90 C. After the reaction began, the rate of CO evolution was to per hour for about /2 hour. The rate then slowly dropped off over a period of 1% hours to about 10% per hour, at which time the reaction was stopped. The yield of adipic acid as determined by vapor phase chromatography analysis of the esterified dibasic acid was mole percent, based on the total moles of unrecovered hexahydrobenzoic acid and cyclohexanone. In addition, glutaric acid was found in the 12 mole percent yield.

It is evident from the foregoing that the process of this invention may not only be used to degrade rings to determine their size and composition, but may also be used preparatively in the preparation of dibasic acids, particularly adipic acid from cyclohexane carboxylic acid. If lower dibasic acids are desired, the adipic acid or other higher dibasic acid may be recycled to the reactor and further degraded as is evidenced by the presence of some glutaric acid along with the adipic acid formed in the oxidative decarboxylation of hexahydrobenzoic acid.

The following examples illustrate a number of acyclic aliphatic monocarboxylic acids which have undergone oxidative decarboxylation according to the process of this invention.

Example XIII In accordance with the procedure described in Example I, 1 mole of pivalic acid was dissolved in 400 cc. of an acetic acid solution, 1/ 10 molal in cobalt'acetate tetrahydrate. The reaction was heated to about to C. and oxygen passed through the reaction mixture at a rate of about 1 mole per mole of pivalic acid per hour.

The reaction was stopped after 2 hours and the reaction mixture analyzed. Among'the products obtained were methanol, acetone, t-butanol and t-butyl peroxide.

Example XIV A soultion of 100 g. of palmitic acid in 500 ml. of glacial acetic acid which was 0.1 molal in cobalt acetate tetrahydrate was charged to a 1,000 ml. turbostirrer, heated to C. and oxygen introduced at a rate of approximately 1.5 mole per mole of palrnitic acid per, hour. The reaction was continued for 21 hours and The temperature was slowly raised to 90 C.

Example XV The oxidation of palrnitic acid was carried out according to the process described in Example XIV, with the exceptions that acetaldehyde was introduced at a rate of approximately 0.1 mole per'mole of palmitic' acid per hour, the length of the reaction was 23 hours and a 58.7% conversion based on recovered starting material was obtained.

The following is a list of the percent yield of fatty acid products obtained based on unrecovered starting material:

C 3.2, C 9.0, C 4.4, C 5.0, Cu 4.7, C 7.3, C 8.9, C 8.9, C 141.

Example XVI The oxidation of palmitic acid was carried out substantially in conformance with the process described in Example XIV with the exceptions that acetaldehyde was introduce-d at a rate of approximately 0.1 mole per mole of palrnitic acid per hour, the temperature was maintained at 85 C. and the length of the reaction was 22 hours. An 86.9% conversion was obtained.

The following is a list of the percent yield of fatty acid products obtained based on unrecovered starting material:

C 2.6, C 4.2, C 3.9, C 4.6, C 5.9, C 7.9, C 9.9, C 11.1, C 195.

Example XVII Into a small turbo-mixer of about 100 to ml. capacity was introduced a mixture containing 40 cc. of glacial acetic acid which was 0.2 molal in cobalt acetate tetrahydrate, 43 cc. of toluene and 25.6 g. of hexahydrobenzoic acid. The mixture was warmed to 95 C. and

end of the time, the solution was cooled to room tem-' perature, the reaction product esterified and analyzed by vapor-phase chromatography. The products based on unrecovered starting material were as follows: adipic acid, 58%; glutaric acid, 9%; cyclohexanone-cyclohexanol-cyclohexylacetate, 24%; and tricarboxylic acid, 4%. The yield of benzoic acid was over 95% as determined by the carbon dioxide balance.

In the above experiment, the benzoic acid could be reduced to hexahydrobenzoic acid and recycled as a reactant back to the reactor. In this way, from toluene one may obtain adipic acid in a two-step process: oxidation of toluene in the'presence of hexahydrobenzoic acid hexahydrobenzoic acid.

to form benzoic acid; reduction of the benzoic acid to In this way one not only has a simple route from toluene to adipic acid, but, because of the presence of the toluene in the oxidation of the hexahydrobenzoic acid, the yields of adipic acid are significantly enhanced.

The process of this invention has a wide variety of applications. It provides a method of obtaining dibasic acids in commercial quantities from readily available cyclic materials, such as hexahydrobenzoic acid. It permits a ready means of co-oxidation with other materials, the products of which may also be of value. This is exor acyclic by the presence or absence of monoor polybasic acids.

As will be evident to those skilled in the art, various modifications on this process can be made or followed, in the light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the following claims.

I claim:

1. A method of oxidative degradation of a monobasic carboxylic acid of 6 to 16 carbons having .an aliphatic alpha carbon. and wherein it has a hydrocarbon group bonded to its carboxyl group which comprises treating said monobasic acid at a temperature in the range of about 25 C. to 150 C. with oxygen as the sole oxidant and a metal salt oxidation catalyst having an atomic number of from 25 to 29, wherein the amount of said catalyst is at least 0.1% by weight based on weight of said monocarboxylic acid.

2. A method according to claim 1 wherein saidmetal salt oxidation catalyst is a cobalt salt oxidation catalyst.

3. A method according to claim 2 wherein said-temperature is in the range of about C. to C.

4. A method of preparing adipic acid which comprises treating hexahydrobenzoic acid with oxygen as the sole oxidant and at least 0.1% by weight based on weight of hexahydrobenzoic acid of a cobalt salt oxidation catalyst at a temperature in the range of 80 C. to 120 C. for a period of time in the range of about /2 hour'to 25 hours and isolating the adipic acid.

5. A method according to claim 4 wherein acetaldehyde in an amount of 0.05 to 0.5 mole per mole of hexahydrobenzoic acid is present in the reaction mixture.

6. A method according to claim 4, wherein the cobalt salt oxidation catalyst is cobalt acetate.

7. A method according to claim 5, wherein the cobalt salt oxidation catalyst is cobalt acetate.

References Cited by the Examiner UNITED STATES PATENTS 2,223,493

LORRAINE A. WEINBERGER, Primary Examiner. LEON ZITVER, Ea'tamin er. 

1. A METHOD OF OXIDATIVE DEGRADATION OF A MONOBASIC CARBOXYLIC ACID OF 6 TO 16 CARBONS HAVING AN ALIPHATIC ALPHA CARBON AND WHEREIN IT HAS A HYDROCARBON GROUP BONDED TO ITS CARBOXYL GROUP WHICH COMPRISES TREATING SAID MONOBASIC ACID AT A TEMPERATURE IN THE RANGE OF ABOUT 25*C TO 150*C. WITH OXYGEN AS THE SOLE OXIDANT AND A METAL SALT OXIDATION CATALYST HAIVNG AN ATOMIC NUMBER OF FROM 25 TO 29, WHEREIN THE AMOUNT OF SAID CATALYST IS AT LEAST 0.1% BY WEIGHT BASED ON WEIGHT OF SAID MONOCARBOXYLIC ACID. 